[fix compatability with ghc 6.10 due to Data.Map.lookup type change
John Meacham <john@repetae.net>**20090218080813
 Ignore-this: 6383f410fc69c2791a2a1184bea3dd70
] hunk ./Util/SetLike.hs 184
-    mlookup = M.lookup
+    mlookup k m = case M.lookup k m of
+        Nothing -> fail "Map: mlookup can't find key"
+        Just x -> return x